As you know I have already pointed out a couple of weird behavior in the past using multiple monitors and also (minor) frustrations with the marco PID just going FUBAR and having to kill it to restore caja.
Most often than not it seems to me that frustrations came back screensaver/energysaver mode when using a 2 monitors setup with both in portrait mode.
Now, this is what happened at the beginning of last week when I faced another similar scenario:
The only way that I can explain this type of GUI behavior up-above to somebody who cannot see those pictures is by explaining that, it seems to me, the picture from the background as taken over from the Z-axis perspective and will be "on top" of any other opened windows in a predefined pixelated area.
Another point to consider in this current state of things is that the top-menu MATE bar has dissapeared and that even tho I killed the marco PID off tty1 that it made me reach that point and my only way of fixing this was to logoff.
Another point I wish to add (but that I have no visuals for) is the following: always on this same system with 2 monitors in portrait mode, while my system was in stand bye mode, I woke it up via mouse/keyboard and on my left most screen screens showed my background and the regular password box field(as it should be) but on the other/right screen: my sublime text editor was fully exposed on top of my background picture.
Possibly 1 second later, there was a "video resize" happening and even tho the password box remained on my left screen, after that "magic moment, my right monitor now had hidden sublime-text alongside the background picture and was back into "energy saving" mode.
Of course, keying in my password brought me back onto my desktop showing everything in normal. But that was weird.
So, yeah. Marco?